1. Field of the Invention
The present invention relates to a machine for processing sheets of printing material. The machine has a cylinder for transporting the sheets and sheet supports that are mounted such that they can be rotated about a sheet support axis of rotation and in each case contain supporting segments for pressing the sheets onto the cylinder.
Sheet-fed presses can have a sheet deliverer which, in the transport direction, contains leading gripper bars for firmly holding leading sheet ends of the sheets to be delivered, and trailing gripper bars for simultaneously firmly holding trailing sheet ends of the sheets. In such a deliverer, the set of leading gripper bars can be fixed to one pair of chains and the set of trailing gripper bars can be fixed to another pair of chains. Together with the pairs of chains, the leading gripper bars and the trailing gripper bars revolve around a delivery drum, which is equipped with the aforesaid sheet supports. The sheet supports are used to press the respective sheet against an impression cylinder adjacent to the delivery drum when the respective leading gripper bar has already gripped the leading sheet and the trailing gripper bars cooperating with the leading gripper bar have not yet gripped the trailing sheet end. The action of pressing the sheet against the impression cylinder, carried out by the sheet supports, is necessary in order that the aforesaid trailing gripper bar can grip the trailing sheet securely. Each of the leading gripper bars runs ahead at a specific distance relative to the trailing gripper bar associated with the aforesaid leading gripper bar. The distance depends on the length of the sheets, which can change from print job to print job. A change in the sheet length requires a correction of the aforesaid distance within the context of what is known as changing the format of the deliverer. During the format change, the leading gripper bar is set to a longer or shorter distance relative to the trailing gripper bar associated with it, by the pair of chains carrying the leading gripper bar being displaced relative to the pair of chains carrying the trailing gripper bar. Since the sheet supports are temporarily located between these two gripper bars during the circulation of the mutually associated leading gripper bars and trailing gripper bars, adaptation of the effective circumferential length of the sheet supports to the changed distance between the gripper bars is necessary within the context of the format change. For instance, the sheet supports have to be shortened if, during the format change, the leading gripper bar has to be displaced toward the trailing gripper bar, in order that the sheet supports do not hamper the displacement of the leading gripper bars.
Published, Non-Prosecuted German Patent Application DE 100 14 417 A1, corresponding to U.S. Pat. No. 6,578,846 and U.S. patent Disclosure 2002/0135123 (therein, see in particular column 9, line 15 to column 10 line 11), in which a machine corresponding to the generic type mentioned at the beginning is described, for this purpose, it is proposed to divide the respective sheet support into a leading sheet support section and a trailing sheet support section. In order to lengthen or shorten the respective sheet support, one of its sheet support sections is displaced in the circumferential direction relative to the other. The sheet support sections form the supporting segments mentioned at the beginning. According to one embodiment (see DE 100 14 417 A1, FIG. 2A), the two sheet support sections are disposed beside each other in such a way that the common track width of the sheet support sections or of their running strips is comparatively large. In this case, it is disadvantageous that the great track width requires a correspondingly great width of the print-free side edge of the sheet, on which the running strips roll. The great width of the print-free sheet side edge results in a restriction of the sheet area available for the printed image, and an increased sheet trim waste volume. In the other embodiments (see DE 100 14 417 A1, FIGS. 2B and 2C), in which the sheet support sections intermesh with one another in some regions and are covered by a carrier belt carrying comparatively narrow-track running strips, these problems are solved but only at the expense of other problems. In those circumferential regions in which the sheet support sections are not interengaged, the carrier belt is carried on the rear side by only one of the sheet support sections in each case and it therefore no longer has sufficient backing and stability. Damage and premature wear of the carrier belt is to be expected.